Servo writing with simultaneous biasing of magneto-resistive read elements

ABSTRACT

Apparatus and method for simultaneously biasing multiple magneto-resistive read elements in a disc drive head/disc assembly. A preamplifier circuit sequentially applies read bias currents to successive pairs of heads of the head/disc assembly, wherein the read bias current applied to a selected one of each pair of the heads is used to transduce the servo data from the associated disc recording surface, and the read bias current applied to the remaining one of each pair of the heads is used to prepare the remaining one of each pair of the heads to subsequently transduce the servo data from the associated disc recording surface. The preamplifier circuit includes at least first and second read bias current sources and a head selection circuit.

RELATED APPLICATIONS

1. This application claims the benefit of U.S. Provisional ApplicationSer. No. 60/088,064, filed Jun. 5, 1998.

2. 1. Field of the Invention

3. This invention relates generally to the field of disc drive storagedevices, and more particularly, but not by way of limitation, toimproving data transfer operations such as servo track writeverification by biasing multiple magneto-resistive read elements of adisc drive.

4. 2. Background of the Invention

5. Hard disc drives are used in modern computer systems to enable usersto store and retrieve vast amounts of data in a fast and efficientmanner. A typical disc drive is generally composed of a head/discassembly (HDA) which houses requisite mechanical portions of the driveand a printed wiring assembly (PWA) which supports requisite electronicportions of the drive.

6. The HDA includes a base deck to which various components are mountedand a top cover which cooperates with the base deck to form a sealedhousing to reduce particulate contamination. Within the housing, a discstack is formed from one or more magnetic recording discs which areaxially aligned for rotation by a spindle motor at a constant, highspeed, such as 10,000 revolutions per minute during normal disc driveoperation.

7. A rotary actuator assembly is mounted adjacent the disc stack andincludes a plurality of rigid arms which extend into the stack betweenadjacent discs, as well as above and below the top and bottom discs. Therigid arms support flexible suspension assemblies which in turn, supporta corresponding number of read/write heads adjacent the surfaces of thediscs. One head is provided for each disc surface to read data from andto write data to the corresponding surface.

8. Of particular interest are magneto-resistive (MR) heads, whichutilize thinfilm inductive write elements to write data and MR readelements to read previously written data. A typical MR read element isformed from an alloy of materials so as to have a baseline electricalresistance which varies in the presence of a magnetic field of aselected orientation. By passing a bias current through the MR element,the selective magnetization of a corresponding track can be determinedin relation to variations in voltage detected across the MR element.

9. A preamplifier/driver circuit is typically mounted to the side of theactuator assembly and includes a write driver circuit to apply writecurrents to a selected write element during a write operation. Thepreamplifier/driver circuit further includes a bias current source whichis sequentially connected to the appropriate read element to effect aread operation.

10. The electronics provided on the disc drive PWA primarily serve tocontrol the operation of the HDA and to communicate with a host computerin which the disc drive is mounted. Generally, the top level functionalblocks oil the PWA include a read/write channel which controls thereading and writing of data from and to the discs, a spindle motorcontrol circuit which controls the rotation of the spindle motor, and aservo control circuit which controls the position of the heads.

11. Aspects of a typical servo control circuit are discussed in U.S.Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee ofthe present invention. The servo control circuit positions the headsrelative to the tracks through the application of current to a coil of avoice coil motor (VCM) within the HDA, the coil being mounted to theactuator opposite the heads. The tracks are defined from servo datawritten to servo data fields on the surfaces of the discs during themanufacturing of the HDA. The servo data are stored as a series ofradially extending servo wedges on each of the disc surfaces, with theservo wedges composed of adjacently aligned servo data fields, with eachservo data field in each wedge defining a unique track. Hence, byperiodically transducing the servo data associated with a particulartrack, the servo control circuit can adjust the current applied to thecoil to adjust the position of the corresponding head. User data fields,which are used to store user data from the host computer, aresubsequently defined between adjacent servo fields during a disc driveformatting operation.

12. Conventionally, the servo data are written using a servo trackwriting system, also commonly referred to as a servo track writer (STW).A typical servo track writer comprises a fixture on which each HDA inturn is mounted. Once mounted, the servo track writer proceeds to writethe servo data using the heads of the HDA. Thus, a typical servo trackwriter includes control circuitry which generally emulates portions ofthe electronics disposed on the PWA, as well as a closed looppositioning system which both detects the radial position of the headsand mechanically advances the heads. Access to the actuator is achievedby providing an opening in the base deck of the HDA which is latersealed.

13. Position detection and feedback are usually carried out using alaser inferometer or other precise optical displacement instrumentation.Mechanical advancement of the heads can be carried out by inserting apusher pin assembly into the HDA to engage and move the actuatorassembly. More recently, positioning systems have also been developedwhich apply current to the actuator coil, thereby utilizing the VCM toadvance the position of the heads. The control and positioning systemsare usually interfaced with a personal computer (PC) which provides agraphical user interface for the STW operator to control the operationof the system.

14. The writing of servo data is an important, but correspondingly timeconsuming, portion of the HDA manufacturing process. A typical STWoperation can take up to several hours per HDA, so that disc drivemanufacturers will often implement large numbers of STW stations toaccommodate large scale disc drive production efforts. For purposes ofefficiency, a servo track writer does not typically write all of theservo data on one disc surface before moving to the next surface;rather, each of the heads are selected in turn so that, after a selectedhead writes a portion of the data at a given radius, the next head isselected and used to write a corresponding portion of the servo data ata selected angular distance from the first portion, and so on. Thisresults in the servo data on each disc surface being offset with respectto the data on adjacent surfaces, resembling steps of a spiralstaircase. This is repeated multiple times around the circumference ofthe discs, as disc drives typically have from 30 to 90 servo wedges oneach surface.

15. Once all of the servo data have been written at a given radius, theservo track writer proceeds to verify the accuracy of the writingoperation. This generally entails sequentially applying a read biascurrent to each of the heads in turn to transduce and verify the servodata from the respective surfaces in the order in which the data wereinitially written to the surfaces. Because only one read bias currentsource is available in the preamplifier circuit, the elapsed timebetween successive servo data fields on successively read data surfacesmust be long enough to allow the current to be switched from the firsthead to the second head and to allow the second head to reach anequilibrium state before the second servo data field is read. Hence, itmay require a reduction in the rotational speed of the disc, oradditional revolutions of the discs, in order to enable the servo trackwriter to verify all of the servo data. This can present a bottleneck ina disc drive manufacturing process, requiring additional investments inresources to meet the required process throughput.

16. Accordingly, there is a continued need for improvements in themanner in which data are tranduced from disc recording surfaces, such asduring the read verification operations carried out during the writingof servo data. It is to such improvements that the present invention isdirected.

SUMMARY OF THE INVENTION

17. The present invention is directed to an apparatus and method forproviding improved read operations upon disc recording surfaces throughthe simultaneous biasing of multiple magneto-resistive read elements.

18. In accordance with preferred embodiments, a disc drive includes ahead/disc assembly (HDA) housing a plurality of disc recording surfacesand an actuator assembly which supports a corresponding plurality ofheads adjacent the surfaces. A preamplifier circuit is mounted to theactuator assembly and includes a first read bias current source whichgenerates a first read bias current of selected magnitude. Thepreamplifier circuit further includes a second read bias current sourcewhich generates a second read bias current of selected magnitudeindependently of the first read bias current. A head selection circuitof the preamplifier circuit selects first and second heads of the HDA sothat the first and second read bias currents are simultaneously directedto the first and second heads, respectively.

19. In this way, a read operation, such as a verification operation usedto verify accuracy of servo data written to the disc recording surfacesduring a servo track write operation, can be carried out efficiently bysequentially applying read bias currents to successive pairs of theheads. The read bias current applied to a selected one of each pair ofthe heads is used to transduce the data from the associated discrecording surface, while the read bias current applied to the remainingone of each pair of the heads is used to prepare the remaining one ofeach pair of the heads to subsequently transduce the data from theassociated disc recording surface.

20. These and various other features as well as advantages whichcharacterize the present invention will be apparent from a reading ofthe following detailed description and a review of the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

21.FIG. 1 is a top plan view of a head/disc assembly (HDA) of a discdrive constructed in accordance with preferred embodiments of thepresent invention, with the HDA combinable with a PWA to complete thedisc drive.

22.FIG. 2 illustrates the manner in which servo data are arranged oneach of the disc surfaces in a plurality of radially extending wedges,each wedge comprising a plurality of adjacently disposed servo datablocks.

23.FIG. 3 shows a preferred configuration of one of the servo datablocks.

24.FIG. 4 provides a functional block diagram for a servo track writerused to write the servo data to the discs of the HDA in accordance withpreferred embodiments.

25.FIG. 5 provides a functional block diagram of a servo control circuitused to effect head positional control by the disc drive, with thecircuitry being disposed on the disc drive PWA and at least portions ofwhich preferably emulated by the servo track writer.

26.FIG. 6 is an elevational view of a disc stack of the HDA, generallyillustrating the manner in which servo data are sequentially ordered onsuccessive disc surfaces.

27.FIG. 7 shows a basic construction of one of the MR heads of the HDA.

28.FIG. 8 provides a representation of a prior art preamplifier/drivercircuit used to supply read bias currents to a plurality of MR heads.

29.FIG. 9 provides a representation of a preamplifier/driver circuithaving multiple read bias current sources to simultaneously apply readbias currents to a plurality of MR heads in accordance with preferredembodiments of the present invention.

30.FIG. 10 provides a representation of an alternative construction forthe preamplifier/driver circuit of FIG. 9 in accordance with preferredembodiments of the present invention.

31.FIG. 11 is a flow chart illustrating a SERVO TRACK WRITE OPERATIONroutine, generally indicative of steps preferably carried out by theservo track writer of FIG. 4 in accordance with preferred embodiments ofthe present invention.

32.FIG. 12 is a flow chart illustrating a VERIFY SERVO DATA subroutinepreferably carried out as part of the routine of FIG. 11 in accordancewith preferred embodiments of the present invention.

DETAILED DESCRIPTION

33. Referring first to FIG. 1, shown therein is a top plan view of adisc drive 100, constructed and formatted in accordance with preferredembodiments of the present invention. The disc drive 100 is formed oftwo primary assemblies: a head/disc assembly (HDA) 101 which composessubstantially all of the mechanical portions of the disc drive, and aprinted wiring assembly (PWA) which supports electronics used to controlthe operation of the HDA. The PWA is mounted to the underside of the HDA101 and is thus not visible in FIG. 1.

34. The HDA 101 includes a base deck 102 to which various disc drivecomponents are mounted. A top cover, which has been omitted from FIG. 1to facilitate the present discussion, cooperates with the base deck 101to form a sealed housing for the HDA 101. A spindle motor 104 isprovided to rotate a stack of discs 106 at a constant high speed duringnormal disc drive operation, with a disc clamp 108 securing the discs tothe spindle motor 104.

35. To access the discs 106, a controllably positionable actuatorassembly 110 (also referred to as an “E-block”) is provided whichrotates about a cartridge bearing assembly 112 in response to currentsapplied to a coil (a portion of which is shown at 113) of a voice coilmotor (VCM) 114. The actuator assembly 110 includes a plurality ofactuator arms from which corresponding flexure assemblies extend, thetopmost of which are identified at 116 and 118 respectively. Heads 120are provided at distal ends of the flexure assemblies 116, 118 and aresupported over the discs 106 by air bearings established by air currentsset up by the rotation of the discs 106. The heads 120 are positionablylocated over data tracks (not shown) of the discs 106 in order to readdata from and write data to the tracks, respectively. As discussed morefully below, the heads 120 are characterized as magneto-resistive (MR)heads, with each head including a thin film inductive write element anda MR read element.

36. A latch assembly 121 secures the heads 120 over texturized landingzones (not shown) at the innermost diameters of the discs 106 when theHDA 101 is not in an operational mode (i.e., when the discs 106 arebrought to rest). A flex circuit assembly 122 facilitates electricalcommunication between the actuator assembly 110 and the PWA. The flexcircuit assembly 122 includes a flex circuit board 124 which supports anencapsulated preamplifier/driver circuit 130 which applies read andwrite currents to the heads 120. Preferred construction and operation ofthe preamplifier/driver circuit 130, also referred to herein as the“preamplifier circuit” and the “preamp,” will be discussed in greaterdetail below.

37.FIG. 2 provides a generalized representation of a preferred manner inwhich servo data are arranged onto the surfaces of the discs 106.Particularly, a number of servo wedges (one denoted at 150) radiallyextend from innermost to outermost radii of recording surface portionsof the discs 106. Each of the servo wedges comprises a plurality ofadjacently positioned and aligned servo data fields, as shown at 152 inFIG. 3.

38. The servo data field 152 includes an automatic gain control (AGC)field 154 which stores an oscillating pattern (such as a 2T pattern) toprepare servo control circuitry (not shown) of the disc drive forreceipt of remaining portions of the servo field 152. A synchronization(sync) field 156 provides timing information to the servo controlcircuitry. Index and Gray code fields 158, 160 respectively, indicatethe angular and radial position of the servo field 152. A position field162 provides inter-track positioning information and serves to definetrack boundaries on the disc surface.

39. The servo data are written to the discs 106 during manufacturing ofthe HDA 101 using a servo track writer (STW) 170, such as generallyrepresented in FIG. 4. The HDA 101 is mounted over a mounting fixture172 which serves as a mechanical reference for the servo track writer170. The mounting fixture 172 typically comprises a granite block havinga calibrated, level top surface on which a mounting plate is placed. Themounting plate includes clamps which locate and secure the HDA 101 tothe mounting fixture 172. As such mounting fixtures 172 are well known,additional discussion will not be provided herein except to state thatthe HDA 101 is mounted in such a manner so as to provide access to theactuator assembly 110 (shown in FIG. 1) by the STW 170 during a servowrite operation.

40. Continuing with FIG. 4, a pusher block assembly 174 is mounted tothe mounting fixture 172 underneath the HDA 101 so as to engage theactuator assembly 10 in order to selectively position the actuatorassembly 1 10 relative to the discs 106. Typically, an opening (notshown) is provided in the bottom of the base deck 102 of the HDA 101 sothat the pusher block assembly 174 can access the internal environmentof the HDA 101 by extending up through the opening in the base deck 102.Once the servo data have been written, the opening is typically coveredby an adhesive label or other means to seal the internal environment ofthe HDA 101.

41. The STW 170 further comprises a positioning system 176 mounted tothe mounting fixture 172 relative to the HDA 101. The positioning system176 rotates the pusher block assembly 174 about a central axis in orderto advance the position of the actuator assembly 110. The position ofthe pusher block assembly 174 is controlled by the positioning system176 through a detection system (not separately designated) which detectsthe position of the pusher block assembly 174 and provides correctionsignals to a motor (also not separately designated) in order to rotatethe pusher block assembly 174 accordingly.

42. A control circuit 178 emulates portions of the disc drive PWA inorder to control the operation of the HDA 101, including the transfer ofservo data to the HDA 101 from a personal computer (PC) 180. The PC 180provides a graphical user interface for a user to control the STWoperation.

43. During operation of the STW 170, the user mounts the HDA 101 to themounting fixture 172 and instructs the STW 170 (by way of the PC 180) tocommence writing the servo data to the discs 106. The control circuit178 instructs the HDA 101 to commence rotation of the discs 106 by wayof the spindle motor 104(FIG. 1) and instructs the pusher block assembly174 to place the actuator assembly 174 at a starting position, such asnear the outer radii of the discs 106. The STW 101 thereafter instructsthe HDA 101 to write the servo data to each of the surfaces of the discs106, while mechanically advancing the pusher block assembly 174 so as todefine each new successive track on the discs 106. Each of the heads 120in turn write the servo data to the corresponding discs 106 at eachincrementally selected disc radius.

44. The time required to complete the writing of the servo data to atypical HDA depends upon the amount of servo data to be written to thediscs, the rotational speed of the discs, the number of individual discsurfaces, and the time required to validate, or verify, that the servodata were written correctly. For reference, it can take several hours tocomplete the STW operation for a single high capacity HDA. Hence, thereare substantial economic benefits to not only writing the data correctlythe first time (to avoid the time and expense of rewriting the data),but also to carrying out all aspects of the STW operation as efficientlyas possible.

45. Before turning to a discussion of preferred methodologies by whichthe STW 170 of FIG. 4 advantageously carries out readback verificationfunctions of the written servo data, reference is made to FIG. 5, whichprovides a functional block diagram of a servo circuit 182 used toeffect head positional control for each of the heads 120 with respect tothe corresponding disc surfaces. It will be recognized that most of theservo circuit 182 is disposed on the aforementioned disc drive PWA andused during normal disc drive operation. However, since the STWoperation is carried out on the HDA 101 before the PWA is mountedthereto, the control circuit 178 preferably includes this circuitry soas to emulate functional aspects of this circuit. Although the STW 170of FIG. 4 utilizes a pusher pin assembly 172 to mechanically advance theactuator assembly 110, the STW 170 can be readily provided with analternative configuration wherein the control circuit 178 utilizescircuitry such as set forth by FIG. 5 to apply current to the actuatorcoil 113 to sequentially advance the actuator assembly 110.

46. As set forth by FIG. 5, servo data are transduced from a selecteddisc surface and, after preamplification by the preamp/driver circuit130 (previously shown in FIG. 1 and herein also referred to as the“preamp”), the servo data are passed to a demodulator circuit (demod)184 which conditions the servo data for presentation to a digital signalprocessor (DSP) 186. During servo circuit position control operations,the DSP 186 operates in accordance with programming stored in DSP memory188 to output current command signals to a coil driver 190, which inturn applies current to the coil 113 to position the head 120 asdesired.

47. Additionally, the DSP 186 communicates with the preamp 130 to selectthe desired head 120 as well as to select the desired bias current forthe head. FIG. 6 shows a schematic representation of a selected head120, generally illustrating the presence of both a write element 192 anda read element 194. As will be understood, the write element 192 is usedto write data to the discs 106 and preferably comprises a thin filminductive element with a conductor wrapped multiple times around ahorseshoe shaped core with an air gap brought into proximity to the discsurface. As pulsed write currents are passed through the conductor,magnetic fringing occurs across the gap, selectively magnetizing thedisc surface.

48. The read element 194 preferably comprises an MR element formed froman alloy of cobalt, nickel and iron and possesses a nominal directcurrent (dc) electrical resistance (such as 40 ohms). When the MRelement is subjected to a magnetic field of selected orientation, theelement undergoes a change in the baseline electrical resistance. Hence,data are read from a disc surface by passing a bias current through theMR element and monitoring changes in voltage thereacross induced by themagnetization of the disc surface. This produces the readback signalwhich can then be decoded by read/write circuitry to reproduce theoriginally stored data. The bias current is applied by the preamp 130and has a magnitude selected by the DSP 186.

49. It will be recognized that all of the heads 120 have the samenominal construction as shown in FIG. 6. Although MR head technology hasbeen presented herein in accordance with preferred embodiments, it willbe further recognized that the present invention, as claimed below, isnot necessarily limited to such construction, but can readily be usedwith other, similar technologies wherein bias currents are applied toread elements to transduce magnetic data, such as giantmagneto-resistive (GMR) and spin-valve technologies.

50.FIG. 7 shows the generally stair-step manner in which servo data arewritten to successive disc recording surfaces. Particularly, FIG. 7 isan elevational representation of a plurality of discs 106 of the HDA 101and corresponding heads 120 which are supported adjacent the surfaces ofthe discs 106 by flexure assemblies 118, as discussed above in FIG. 1.The servo wedges 150 (FIG. 2) are angularly displaced as shown so that,as discussed above, the heads 120 are cycled to write the servo data atdifferent angular locations while being maintained at the same radiallocation with respect to the discs 106.

51.FIG. 8 shows a generalized functional block diagram of relevantportions of a prior art preamplifier/driver circuit (“preamp”) 200 whichis operably coupled to a total of four read/write heads 202, 204, 206and 208. The heads 202, 204, 206 and 208 are nominally identical to theheads 120 discussed above, each thereby possessing an MR construction.It will be understood that the use of four heads is merely for purposesof illustration, in that prior art preamps such as 200 are commerciallyavailable to service a wide variety in the number of heads (such as upto 20 heads). Moreover, it will be understood that the preamp 200further includes circuitry used to apply write currents to the writeelements 192 (FIG. 6) of the heads, but such has been omitted forclarity.

52. The functional blocks of the prior art preamp 200 set forth in FIG.8 include a buffer 210, a bias current source 212, a head selectioncircuit 214, switching circuitry represented as a multiplexer (mux) 216,and a read amplifier (amp) 218. During operation, a multi-bit input wordis supplied to the buffer 210 which indicates, among other things, adesired head to be selected and a desired magnitude of bias current tobe applied thereto. This input word is input along path 220 and can besupplied, for example, by a DSP such as illustrated at 186 in FIG. 5.

53. In response to the input word, the bias current source 212 selectsand outputs the appropriate read bias current along path 222 to the mux216. At generally the same time, the head selection circuit 214 uses theinput word to select (via path 224) the desired head from the populationof heads 202, 204, 206 and 208 (for example, the top head 202). In thismanner, the bias current source 212, head selection circuit 214 and mux216 cooperate to pass the desired magnitude of read bias current throughthe MR element of the selected head 202. As the head 202 transduces theselective magnetization of the corresponding disc surface, variations inthe voltage across the MR element of the head 202 are sensed andamplified by the read amp 218 to output an amplified read signal on path226. The amplified read signal is thereafter decoded to reconstruct thepreviously stored data.

54. While operable, the prior art preamp 200 of FIG. 8 presentslimitations in the ability to transduce data from multiple heads. Forexample, switching from the first head 202 to the second head 204requires the input of a new input word to the buffer 210, the decodingof this word by the bias current source 212 and the head selectioncircuit 214, the adjustment of the magnitude of the bias current to thenew level and application of the new bias current magnitude to thesecond head 204, and the thermal stabilization of the second head 204.The foregoing actions are necessary before data can be transduced usingthe second head 204. Thus, these and other considerations limit thespeed at which data can be transduced from successively selected discsurfaces. In the context of a STW operation, the rotational speed of thediscs 106 may need to be slowed, or the servo wedges 150 spacedsufficiently apart on successive surfaces, to allow these actions to becarried out without requiring multiple revolutions of the discs 106.

55. Accordingly, FIG. 9 provides a functional block diagram of relevantportions of the preamp 130, constructed in accordance with preferredembodiments of the present invention. For ease of illustration, thepreamp 130 is contemplated as being connected to the four heads 202,204, 206 and 208 discussed in FIG. 8. It will be understood, however,that the present invention as claimed below is not limited to theparticular configuration shown in FIG. 9.

56. As with the prior art preamp 200 of FIG. 8, the preamp 130 of FIG. 9generally includes functional blocks that carry out bias currentgeneration, head selection, switching and read signal amplification.Significantly, however, the preamp 130 includes multiple read biascurrent sources (identified at 232 and 234, respectively), each of whichindependently and simultaneously outputs a bias current of a selectedmagnitude.

57. Generally, during operation a multi-bit word is provided to a buffer236, indicative of not only the presently desired head to be selected(such as the first head 202) and the associated magnitude of biascurrent to be applied thereto, but also of the next head to besuccessively selected (such as the second head 204) and the associatedmagnitude of bias current for the next head. In response to the inputword, the bias current sources 232, 234 each select and output therespective magnitudes of bias current on paths 238, 240 respectively, tofirst, second and third multiplexers (muxs) 242, 244 and 246, as shown.It will be noted that the first and second muxs 242 and 244 are eachconnected to each of the heads 202, 204, 206 and 208. Although theseinterconnections are shown to be outside the dotted line boxrepresenting the preamp 130, it will be understood that theseinterconnections are contemplated as being formed within the preamp 130so that only two conductors are preferably extended from the preamp 130to each MR read element of the heads 202, 204, 206 and 208.

58. A head selection circuit 248 is operably coupled to the buffer 236and the first and second muxs 242, 244 to channel the first and secondbias currents (from the first and second bias current sources 232, 234)to the appropriate heads; in this example the first and second heads 202and 204, respectively. Thus, the preamp 130 advantageously operates tosimultaneously apply read bias currents to multiple heads. However, asthe data transduced from the discs 106 is output serially (i.e., onedisc at a time), the third mux 246 switches between the first and secondheads 202, 204 by a selection input on path 250 to connect theappropriate readback signal to the read amp 218. This results in thegeneration of an amplified readback signal on path 226, as before. Theselection input on path 250 can be provided, for example, by a sequencer(not shown) of the read channel circuitry which outputs read gatesignals indicative of times when data are expected to be received fromthe preamp 130.

59. In this way, with reference again to FIG. 7, while the servo dataare verified on the top surface of the top recording disc 106, theappropriate read bias current is simultaneously applied to the next headso that, when the servo data on the bottom surface of the top recordingdisc 106 reaches the next head, the next head is ready to transduce thisnext servo data. This operation continues to each successive head inturn, allowing efficient read verification of the servo data. It will benoted that the preamp 130 is particularly suited for operation duringservo write verification, since the selection of heads occurs inaccordance with a predetermined order.

60. Hence, with reference again to FIG. 9, it will be noted that in onepreferred embodiment, the buffer 220 receives successive input wordswhich allow the head selection circuit 248 and the bias current sources232, 234 to sequentially apply the desired bias currents to the desiredheads in the desired order. In another preferred embodiment, the preamp130 is configured to operate in a special STW mode (set by a particularbit in the input word on path 220), which instructs the preamp 130 toautomatically cycle among a series of predetermined head/bias currentcombinations suitable for a particular STW operation.

61.FIG. 10 provides an alternative preferred construction for the preamp130, similar to that set forth by FIG. 9 except that each of the heads202, 204, 206 and 208 is provided with a separate bias current source(identified at 252, 254, 256 and 258, respectively). Again, only fourheads have been shown in FIG. 10 for purposes of illustration, but theclaimed invention is not so limited.

62. A head selection circuit 260 operates to selectively direct biascurrents of selected magnitude from the bias current sources 252, 254,256 and 258 to the heads 202, 204, 206 and 208, respectively. Thevoltages across the heads 202, 204, 206 and 208 are presented to asuitable switching network, such as a multiplexer 262, to sequentiallyapply these voltages to the read amp 218. Although it is contemplatedthat all of the bias current sources 252, 254, 256 and 258 could besimultaneously applied to the heads 202, 204, 206 and 208, suchoperation could undesirably increase the power requirements (and hence,the heat dissipation) of the preamp 130.

63. Hence, the head selection circuit 260 preferably operates to directbias currents to only two heads at a time: the head that is presentlybeing used to transduce data and the next head in line to transducedata. Once each head in turn has finished reading a particular servodata field 152 (FIG. 3), that head is deselected in favor of the nexthead in line. For example, the head selection circuit 260 operates toapply read bias currents to the first and second heads 202 and 204;after the data are transduced by the first head 202, the preamp 130proceeds to transduce the data by the second head 204 while applyingread bias current to the third head 206, and so on. As before, it iscontemplated that the head selection circuit 260 can operate in responseto input words on path 220, or can be configured to operate in a specialSTW mode to enable automatic cycling of the heads in a predeterminedorder. Selection inputs on path 250 can be used to indicate times whendata are expected to be received from the preamp 130.

64.FIGS. 11 and 12 summarize preferred operation of the preampconfigurations of FIGS. 9 and 10 during STW operations. As discussedabove, the preamp configurations of FIGS. 9 and 10 are particularlysuited to servo track write verification operations, since the order inwhich the heads are to be successively selected are predetermined inrelation to the arrangement of the servo data on the disc surfaces. Itwill be appreciated, however, that to the extent that it is known whichhead is to be selected next during normal disc drive operations, thepreamp configurations of FIGS. 9 and 10 can be also used in a similarfashion to improve data transfer performance.

65. Beginning with FIG. 11, shown therein is generalized flow chart fora SERVO TRACK WRITE OPERATION routine 300, generally indicative of stepscarried out by the STW 170 of FIG. 4 in accordance with preferredembodiments.

66. At step 302, the HDA 101 is first mounted to the mounting fixture172 (FIG. 4) and necessary preparations are made to write the desiredservo data to the HDA 101. Such preparations can include the insertionof a clock head (not shown) into the base deck 102 in order to write aclock track to an outer radius of a selected disc 106 in order toprovide timing information during the STW operation.

67. Next, appropriate magnitudes of read bias and write currents areselected at step 304. As will be recognized, although MR heads arenominally identical, each will generally provide optimal performance atslightly different magnitudes of read bias and write currents. Thusconventional operations can be first carried out to select appropriatecurrent magnitudes for each of the heads; alternatively, based onhistorical data, it may be determined that the servo data can beadequately written to and transduced from each of the disc surfacesusing the same magnitudes of currents for each head.

68. At step 306, the STW 170 proceeds to position the actuator assembly110 to the desired location so that the heads are ready to commencewriting, which occurs at step 308. More particularly, the STW 170 (FIG.4) operates to write the desired servo data to a selected radiallocation on the discs 106, cycling through each of the disc surfaces asillustrated in FIG. 7. Although STW operations can vary, it is common towrite the servo data in multiple increments for each track (such as inone-half track increments) so that multiple passes are made to completeall the servo fields 152 (FIG. 3) in a given cylinder (i.e., all trackson all the discs 106 at a given radius). Hence, in a preferredembodiment one complete cylinder of servo fields 152 are written duringthe step 308 before readback verification takes place.

69. The servo data are next verified at step 310, which is set forthmore fully by the flow of FIG. 12, discussed below. Once theverification step is completed, decision step 312 determines whether allof the servo data have been written to the discs 106; if not, theroutine continues to step 314 wherein the head position is incrementedto prepare the heads to write the next cylinder of servo data, and theroutine returns to step 308. Once all of the servo data have beenwritten to the discs, the routine passes from the decision step 312 toend at step 316.

70.FIG. 12 provides a flow chart for a VERIFY SERVO DATA subroutine,preferably constituting steps carried out by the step 310 of FIG. 11.

71. At step 318, the routine operates to select a pair of headsidentified as the “present head” and the “next head,” with the presenthead comprising the head from which servo data are to be transduced andthe next head comprising the next head in line for use in transducingservo data. In accordance with the foregoing examples of FIGS. 9 and 10,the present head would initially be the first head 202 and the next headwould initially be the second head 204. During step 318, the appropriatebias currents are simultaneously applied to the present head and thenext head, in that the next head receives read bias current while thepresent head receives read bias current.

72. Next, the present head is used to transduce the servo data on thecorresponding disc surface, as indicated by step 320. As before, areadback signal is generated as the servo data pass under the presenthead, with this readback signal being amplified by the read amp 218 andoutput on path 226. During the operation of step 320, any servo datafields 152 having errors are logged for subsequent rereading and, ifsuch persist, appropriate corrective actions. Once the present head hascompleted reading the servo data, the present head is deselected at step322. This entails the removal of the read bias current from the presenthead by the preamp 130.

73. Decision step 324 next determines whether all of the servo data atthe selected position of the heads have been verified; if not, thepresent head and the next head are incremented at step 326. For example,if the heads are identified as K₀ through K_(n) and the present head isinitially set to head Ko and the next head is initially set to K₁, thenthe operation of step 326 serves to adjust the present head to K₁ andthe next head to K₂. The subroutine continues in like fashion until allof the servo data at the selected head location (such as the selectedcylinder) have been verified. For example, if there are 90 servo wedges150 (FIG. 2) on each disc surface, then the subroutine 310 will cycleback through 90 times to read verify each of the associated servo datafields 152. It is contemplated that this operation can be readilycarried out during one revolution of the discs 106. Once all of theservo data have been verified, the routine returns at step 328.

74. In view of the foregoing, it will be recognized that the presentinvention is directed to an apparatus and method for simultaneouslybiasing multiple magneto-resistive read elements in a head/disc assembly(HDA) of a disc drive.

75. In accordance with preferred embodiments, an HDA 101 includes apreamplifier circuit 130 having a first read bias current source 232,252 which generates a first read bias current of selected magnitude anda second read bias current source 234, 254 which generates a second readbias current of selected magnitude independently of the first read biascurrent. A head selection circuit 248, 260 selects first and secondheads 202, 204 of the HDA disc drive so that the first and second readbias currents are simultaneously directed to the first and second heads,respectively.

76. For purposes of the appended claims, the phrase “disc drive” will beunderstood consistent with the foregoing discussion to describe a datastorage device of the type used to store computerized data, such as 100.The phrase “head/disc assembly,” as denoted above by reference numeral101, will be understood to describe a mechanical assembly of a discdrive housing one or more rotatable discs 106 and an actuator assembly110 which supports a plurality of heads 120, 202, 204, 206, 208 adjacentthe discs 106. The term “magneto-resistive” will be understood to coverhead construction technologies which utilize read bias currents totransduce magnetically stored data.

77. It will be clear that the present invention is well adapted toattain the ends and advantages mentioned as well as those inherenttherein. While a presently preferred embodiment has been described forpurposes of this disclosure, numerous changes may be made which willreadily suggest themselves to those skilled in the art and which areencompassed in the spirit of the invention disclosed and as defined inthe appended claims.

What is claimed is:
 1. A method for performing a read operation in ahead/disc assembly of a disc drive, comprising steps of: (a) applying afirst read bias current to a first head of the head/disc assembly totransduce data from a corresponding disc recording surface; and (b)simultaneously applying a second read bias current to a second head ofthe head/disc assembly.
 2. The method of claim 1 , further comprising astep of: (c) using the second read bias current to transduce data from acorresponding disc recording surface while simultaneously applying athird read bias current to a third head of the head/disc assembly. 3.The method of claim 1 , wherein the read operation comprises a writeverification operation and the data comprises servo data.
 4. Apreamplifier circuit for applying read bias currents tomagneto-resistive heads of a head/disc assembly of a disc drive toobtain readback signals in relation to selective magnetization of discrecording surfaces adjacent the heads, comprising: a first read biascurrent source which generates a first read bias current of selectedmagnitude; and a second read bias current source which generates asecond read bias current of selected magnitude independently of thefirst read bias current, wherein the preamplifier circuit simultaneouslyoutputs the first and second read bias currents.
 5. The preamplifiercircuit of claim 4 , further comprising a head selection circuit,operably coupled to the first and second read bias current sources,which selects first and second heads of the head/disc assembly so thatthe first and second read bias currents are simultaneously directed tothe first and second heads, respectively.
 6. The preamplifier circuit ofclaim 5 , further comprising a read preamplifier, operably coupled tothe first and second read bias current sources, which amplifies firstand second readback signals successively generated in relation to theselective magnetization of first and second recording surfaces adjacentthe first and second heads, respectively.
 7. The preamplifier circuit ofclaim 5 , wherein the head/disc assembly further comprises a third head,and wherein the first read bias current source is subsequently coupledto the third head to apply a read bias current of selected magnitude tothe third head while the second read bias current remains directed tothe second head.
 8. The preamplifier circuit of claim 5 , wherein thehead/disc assembly further comprises a third head, wherein thepreamplifier circuit comprises a third read bias current source whichgenerates a third read bias current of selected magnitude independentlyof the first and second read bias currents, and wherein the third readbias current source is subsequently coupled to the third head to applythe third read bias current to the third head while the second read biascurrent remains directed to the second head.
 9. A head/disc assembly ofa disc drive, comprising: a plurality of rotatable disc recordingsurfaces on which data are stored; a plurality of heads adjacent thedisc recording surfaces, wherein each head is provided with amagneto-resistive read element to transduce the data from acorresponding disc recording surface as a read bias current is appliedto the read element, the plurality of heads including at least first andsecond heads; and a preamplifier circuit, operably coupled to the heads,which simultaneously applies first and second read bias currents to thefirst and second heads, respectively, while outputting a first readback,signal indicative of data transduced by the first head in response tothe first read bias current.
 10. The head/disc assembly of claim 9 ,wherein the preamplifier circuit subsequently outputs a second readbacksignal indicative of data transduced by the second head in response tothe second read bias current.
 11. The head/disc assembly of claim 9 ,wherein the plurality of heads further includes at least a third head,and wherein the preamplifier circuit subsequently applies a third readbias current to the third head while the second read bias currentremains directed to the second head.
 12. A head/disc assembly of a discdrive, comprising: first and second heads supported adjacentcorresponding first and second disc recording surfaces; and bias meansfor simultaneously applying first and second read bias currents to thefirst and second heads.
 13. A method for writing servo data to ahead/disc assembly of a disc drive, comprising steps of: (a) mountingthe head/disc assembly to a servo track writer, the head/disc assemblycomprising a plurality of heads supported adjacent a correspondingplurality of disc recording surfaces; (b) using the heads to write servodata at a selected radial location on each of the disc recordingsurfaces, wherein the servo data on each of the disc recording surfacesis angularly offset with respect to the servo data on adjacent discrecording surfaces; and (c) performing a write verification operation toverify accuracy of the servo data by sequentially applying read biascurrents to successive pairs of the heads, wherein the read bias currentapplied to a selected one of each pair of the heads is used to transducethe servo data from the associated disc recording surface and the readbias current applied to the remaining one of each pair of the heads isused to prepare the remaining one of each pair of the heads tosubsequently transduce the servo data from the associated disc recordingsurface.
 14. The method of claim 13 , wherein the head/disc assemblycomprises a preamplifier circuit operably coupled to the heads andcomprising: a first read bias current source which generates a firstread bias current of selected magnitude; and a second read bias currentsource which generates a second read bias current of selected magnitudeindependently of the first read bias current, wherein the preamplifiercircuit simultaneously outputs the first and second read bias currents.15. The method of claim 14 , wherein the preamplifier circuit furthercomprises a head selection circuit, operably coupled to the first andsecond read bias current sources, which selects each successive pair ofheads so that the first and second read bias currents are simultaneouslydirected to each successive pair of heads.
 16. Apparatus for writingservo data to a plurality of disc recording surfaces of a disc drivehead/disc assembly, comprising: a positioning system, operably coupledto the head/disc assembly, which controls radial position of heads ofthe head/disc assembly with respect to the disc recording surfaces; acontrol circuit, operably coupled to the heads, which controls thewriting of the servo data to the disc recording surfaces; and apreamplifier circuit, operably coupled to the heads, which sequentiallyapplies read bias currents to successive pairs of the heads, wherein theread bias current applied to a selected one of each pair of the heads isused to transduce the servo data from the associated disc recordingsurface and the read bias current applied to the remaining one of eachpair of the heads is used to prepare the remaining one of each pair ofthe heads to subsequently transduce the servo data from the associateddisc recording surface.
 17. The apparatus of claim 16 , wherein thepreamplifier circuit comprises: a first read bias current source whichgenerates a first read bias current of selected magnitude; and a secondread bias current source which generates a second read bias current ofselected magnitude independently of the first read bias current, whereinthe preamplifier circuit simultaneously outputs the first and secondread bias currents.
 18. The apparatus of claim 17 , wherein thepreamplifier further comprises a head selection circuit, operablycoupled to the first and second read bias current sources, which selectseach successive pair of heads so that the first and second read biascurrents are simultaneously directed to each successive pair of heads.